Append a new action at the end. The cmd variable must be a valid bourne shell
command. The kind variable consists of two letters.

The first letter can be either of '-' (which means the command reads its
standard input), 'f' (which means the commands reads a given file on the
command line) or '.' (which means the commands reads no input, and hence
must be first.)

Similarly, the second letter can be either of '-' (which means the command
writes to standard output), 'f' (which means the command writes a file on
the command line) or '.' (which means the command does not write anything,
and hence must be last.)